1. Field of the Invention
The present invention relates to an optical module and more particularly to a substrate structure in which an optical element, an optical waveguide and an electrical element are all present. Further, the present invention relates to a manufacturing method thereof.
2. Description of the Related Art
A substrate wherein an optical element, an optical waveguide and an electrical element are all present, has, hitherto, a structure in which an optical waveguide is formed on a multi-layered ceramic substrate and, thereon, an optical element and an electrical element are mounted, as described, for example, in Japanese Patent Application Laid-open No. 236731/1997. In such a structure, however, both the optical element and the electrical element are directly exposed to the air so that the optical element and the electrical element are liable to be degraded under the influence of oxygen and moisture in the air. Therefore, for practical application, this structure by itself is insufficient, and there is required an additional arrangement to cut off the air from this substrate, such as enclosing the substrate in a package with hermetic seal or the like. In this instance, a package structure must be designed in such a way that the optical signal and the electrical signal can be brought out from the package structure without affecting the airtightness therein. This complicates the structure and increases in number of components and steps for assembly and, thus, causes a disadvantage of high cost.
To overcome this, for example, in Japanese Patent Application Laid-open No. 61676/1997, there is disclosed a method in which, on the surface of a substrate where an optical waveguide is formed, two recess sections are formed and an optical element and an electrical element are mounted therein, respectively, and, by covering them with a lid and fixing the lid with resin, the optical element and the electrical element are isolated from the air. Further, another method to isolate an optical element and an electrical element from the air is also disclosed therein. In this method, instead of using a lid, recess sections on which the optical element and the electrical element are mounted are filled up with epoxy resin. Referring to FIGS. 4(a) and 4(b), this method is disclosed below. FIG. 4(a) is a plan view and FIG. 4(b) is a cross-sectional view, taken on line A-Axe2x80x2 of FIG. 4(a), of a substrate into which an optical element and an electrical element are incorporated. As shown in FIGS. 4(a) and 4(b), a lower clad 3 of optical waveguide, cores 4 of optical waveguide and an upper clad 5 of optical waveguide are formed on a substrate 1. In the upper clad 5 of optical waveguide, two recess sections 8 are formed by means of microfabrication.
Within one recess section 8, an optical element 6 is fixed, being aligned precisely to the position of the cores 4 of optical waveguide, and, within the other recess section 8, the electrical element is mounted. The electrical connection between the optical element 6 and the electrical element 7 is made by connecting electrodes 11 on which the optical element 6 is placed to terminals 12 of the electrical element 7 with bonding wires 10. The inside of these recess sections 8 where the optical element 6 and the electrical element 7 are mounted is filled up with epoxy resin 9, and thereby the elements are isolated form the air.
However, the above methods, in which recess sections are formed on the substrate surface and an optical element and an electrical element are mounted therein, and then either a lid is laid thereon or the inside of the recess sections is filled up with resin, have the following problems.
Firstly, minute recess sections capable of containing an optical element or an electrical element must be formed on the substrate surface. In order to form such recess sections, microfabrication technology is necessary and, in consequence, the steps of fabrication process becomes complicated and the production cost increases.
Secondly, the optical element and the electrical element must be mounted within minute recess sections. Especially, for mounting of the optical element, a precision better than 1 xcexcm with respect to the position of the optical waveguide is required. Such a high-precision mounting is a very difficult task to perform within a minute recess section and, in order to carry out that, costly mounting apparatus with a specifically designed mechanism is needed.
Thirdly, a step of making a structure to isolate the optical element and the electrical element from the air and a member for that step become necessary. That is to say, in the method wherein the optical element and the electrical element within recess sections in the substrate are covered with a lid., steps of placing the lid thereon and fixing the lid with resin are additionally required and the lid and the resin for fixing the lid are additionally required members for them. On the other hand, in the method wherein the inside of the recess sections in the substrate where the optical element and the electrical element are mounted is filled up with epoxy resin, steps of applying the epoxy resin thereto and hardening the epoxy resin are additionally required and the epoxy resin is an additionally required member. Such an additional requirement of any step and member directly leads to an increase in production cost.
A principle object of the present invention is to provide, for a substrate on which an optical element and an optical waveguide are disposed with their positions aligned precisely, a structure to isolate, at least, the optical element from the air, with low cost.
The present invention relates to an optical module wherein a substrate on which an optical waveguide is formed and, at least, an optical element is mounted, has a structure in which an upper clad of optical waveguide covers the optical element completely and thereby cuts off the air therefrom.
Further, the present invention relates to a method of manufacturing an optical module having a substrate on which an optical waveguide is formed and, at least, an optical element is mounted; which comprises steps of
forming a lower clad and a core of optical waveguide on the substrate,
mounting the optical element on the substrate so as to align an optical axis thereof to the core, and,
forming an upper clad of optical waveguide to cover the optical element completely.
In this manner, as shown in FIG. 1, the upper clad 5 of polymer optical waveguide covers the surface of the optical element 6 and, if required, that of the electrical element 7 so that the optical element 6 and the electrical element 7 do not come into contact with the air, which produces the effect of protecting both elements from degradation by oxygen or moisture in the air.
Further, in the manufacturing steps of the present invention, the surfaces of the optical element 6 and the electrical element 7 can be covered, simultaneously with the formation of the upper clad of optical waveguide, without requiring any special extra step or member to isolate the optical element 6 and the electrical element 7 from the air. This simplifies the steps and members, and produces another effect of lowering production cost.
The present invention successfully achieves the isolation of, at least, an optical element from the air, on the basis of a standard arrangement in which, simultaneously with the formation of an upper clad of polymer optical waveguide, at least, an optical element and preferably both an optical element and an electrical element are covered with the upper clad of optical waveguide. Therefore, in order to isolate an element from the air, no special extra manufacturing steps or members are required and, thus, production cost can be reduced.
In the present invention, especially since mounting of the optical element in which high precision is required for making alignment with the optical waveguide is performed prior to the formation of the upper clad of optical waveguide, the formation of minute recess sections on the upper clad becomes unnecessary. Moreover, because mounting within such a minute recess section is not needed any more, alignment with a high precision can be more easily achieved than in the conventional method.